The present invention concerns the measurement of optical properties of matter and in particular it relates to instruments which measure such properties as turbidity, absorption, reflection, fluorescence and phosphorence of a fluid sample or of a processed fluid, or which measure such properties (except turbidity) at a solid interface.
Instruments for the detection of these properties are well known but they have tended to be of a form suitable for use in a laboratory under specified conditions (to reduce ambient light disturbances). That is, instruments of the type of which the sample is taken to a test cell, or designed for specific industrial uses in situ. Designs are limited because of the difficulties involved in getting light to the point at which the test is carried out and in gathering the light resulting from the test operation in a prescribed precise manner for subsequent measurement. The advent of fiber-optics has ameliorated some of the problems of conveying light to and from a test site, but a single versatile system that has the performance of a highly specialized instrument and yet can be applied equally well to process control, on-site industrial measurement, in-vivo measurement, e.g. as a medical tool, or measurement under laboratory conditions has escaped the industry. Moreover, no single versatile instrument for use even under adverse lighting conditions and for inaccessible locations and/or adverse or hazardous environments, has become available.
The availability of such instruments has escaped the industry for many technical reasons. Primarily, the characteristics of flash light emission, electronic detector gating and fiber-optics combine to defeat the competitive accuracy of 1% which is achievable with conventional bulk sample spectraphotometers. Turning first to characteristics of light emission as, for example, with a Xenon flash tube, it is elementary understanding that the arc position within the tube varies with each flash. A tube element generally incorporates oppositely facing, parallel electrodes which permit the formation of an arc therebetween upon energizing. The specific relative location and therefore the amplitude and frequency of the arc during each flash cycle varies depending on where the arc strikes the electrode. Thus, the apex of the light cone emanating from the lamp randomly changes position with each flash which results in changes of the amplitude and frequency of emitted light. Consequently, these changes induce loss of specific repeatability and a corresponding diminution of detectability in a fixed detector system.
Detection of a large quantity of light passing through a bulky sample is not seriously affected by the foregoing consideration. However, the performance of a fiber optic-based detector is considerably impaired. Fiber-optics provide a target requiring a very specific geometry of light impingement in order to convey the light signal. Essentially, an optical fiber, conventionally having a diameter of 4 mm, has a limited cone of acceptance which must be substantially aligned with the source light cone in order to achieve sufficient detection. Bearing in mind restrictive optical characteristics of the optical fibers (refraction in "the light pipe"), it has been determined that the angle of the cone of acceptance is directly related to the light frequency. If light in the ultraviolet range is employed, the cone angle approaches 20.degree. and if, visible, 50.degree.. Accordingly, if the apices of the light cone and the cone of acceptance are not aligned within these ranges, the fiber will not "see" the light.
Where it is desired to couple a variable flash source such as a Xenon tube with a fiber-optic detector, the engineering and design must contemplate the limitations described above. Should they be ignored, the coupling back or return of the signal both in amplitude and spectral response, generated by the illuminated sample, is prevented or seriously impaired. Therefore, little or no detection is achieved.
Referring to a specific example of a Xenon flash tube in a spectrophotometer, Hutchins, U.S. Pat. No. 3,810,696, describes an instrument in which a flash is directed through a bulk sample containing cell where light transmission is detected by a dual photodetector. When employing the specific Xenon tube recited, by specification, light amplitude varies by no less than 10% and the incidence of the light flash ("timejitter" - cyclic repeatability) exceeds 200 nano seconds. No consideration is given to the use of a fiber-optic detector. Thus in addition to the foregoing problem, Hutchins, being representative of the prior art, fails to consider the requirement for alignment of the apices. Accordingly, unless alignment occurs by chance, no "light pipe" is enabled.
Taking into account the random placement of the arc, the limited cone of acceptance with fiber-optics, as well as the frequency relationship, the probability of achieving detection with a conventional device such as Hutchins, would be limited to minimal statistical chance --wholly unsatisfactory for purposes of reliable, repeatable measurement.
Adding to the foregoing problems, the art fails to properly account for the proportion of "real" light detected from the sample. Hutchins, exemplary of conventional systems, triggers or gates the detector only after light output attains a particular threshold. Generally, this threshold is based on either a particular fixed time after flash initiation or a fixed intensity of light transmitted. Combining fluctuations in source-light intensity and amplitude with this threshold concept of triggering, leads to a system which virtually defies repeatability. As a result of the proportionality of light detection based in pulse width and amplitude (which varies at minimum up to 10%) the prior art fails to achieve a system allowing for 1% detection accuracy. Especially when considering measurement of weak light signals from dilute samples, modification of a conventional detector such as Hutchins' would require considerations of source and detector optics alignment, as well as gating, neither being recognized.